In addition to such optical recording media as Compact Disc Read-Only Memory (CD-ROM), recordable CDs (CD-Rs and CD-RWs) are now commercialized and widely used.
In contrast to conventional CDs, CD-Rs and CD-RWs feature that the user can write to them and signals generated following recording are designed to meet the conventional CD standards so that they can be played on CD players available on the market.
As a method of achieving fabrication of such recording media, for example, Patent Literature 1 discloses a method of manufacturing a CD-R disc by applying a dye over a substrate by spin coating to form a light-absorbing layer and providing a metallic reflective layer on the light-absorbing layer.
Dye-based materials are used as the materials of the light-absorbing layer.
One feature of CD-R is its high reflectance (65%), which meets the CD standards. For the purpose of obtaining high reflectance with the foregoing disc configuration, the light-absorbing layer needs to have a specific complex refractive index over a wavelength range of the laser beam source for recording and reproduction, and utilization of dye's light absorption property is suitable for achieving this.
As shown in FIG. 1, however, there is a problem that the complex refractive index is greatly dependent on the wavelength because it utilizes the feature of the absorption band end in the light absorption spectrum of the dye film.
As next-generation recording media, DVD-R and DVD+R, are commercialized that are capable of high-density recording and/or reproduction by using a laser beam with a wavelength of around 650 nm rather than using a conventional laser beam with a wavelength of around 780 nm designed for CD. The performance of recording material containing the foregoing dye for CD-R (e.g., pentamethine cyanine dyes or phthalocyanine dyes) is largely dependent on the wavelength of the laser beam; therefore, it results in failure to satisfy recording/reproduction characteristics at 650 nm. This is due to a large absorption coefficient (k) at 650 nm and low reflectance, which makes information recording/reproduction difficult.
To overcome this problem, organic dyes such as trimethine cyanine dyes, azo dyes and tetra-azaporhirazine are proposed as materials for recoding layer which adjust the range of the absorption wavelength of dyes or recording materials. These dyes are disclosed for instance in Patent Literatures 2, 3, 4, and 5.
Along with a recent increase in the recording velocity for recordable DVD discs, improvements are made also on recording materials; examples are squaric acid complexes and trimethine cyanine dye materials, which are disclosed for instance in Patent Literatures 6, 7, and 8.
Even with these recording materials, however, it is difficult to achieve high-speed recording characteristics and archivability (durability) at the same time. The reason for this is that although recording materials that exhibit low thermal decomposition temperatures and high decomposition rates are preferred, these materials are likely to cause a reduction in heat resistance and wet resistance. Generally, two or more dye materials are mixed to avoid this, which however leads to the problem described below.
While a dye recording film is deposited by application of its coating solution by spin coating, the coating solution being obtained by dissolving a dye in solvent, the combined use of two or more different recording materials—for example, a coating solution containing a cyanine dye with ionic structure and a squalene compound with chelate structure—is more likely to cause interactions such as exchange reactions in the mixture, resulting in failure to obtain desired characteristics due to chemical changes.
In particular, the PF6− ion, an anion in the cyanine dye, is so reactive that it tends to cause dechelation of squaric acid complex, for example.
(Patent Literature 1) Japanese Patent Application Laid-Open (JP-A) No. 02-42652
(Patent Literature 2) Japanese Patent (JP-B) No. 2594443
(Patent Literature 3) Japanese Patent Application Laid-Open (JP-A) No. 09-169166
(Patent Literature 4) Japanese Patent Application Laid-Open (JP-A) No. 09-66671
(Patent Literature 5) Japanese Patent Application Laid-Open (JP-A) No. 11-48612
(Patent Literature 6) International Publication No. WO2002/050190
(Patent Literature 7) Japanese Patent (JP-B) No. 3698708
(Patent Literature 8) Japanese Patent (JP-B) No. 3659922